Edible lipids have been manufactured industrially from old times. For example, in the case of soybean oil which is most abundantly manufactured at present, after washing soybean seeds, outer skins thereof are removed if necessary and then the soybean seeds are crushed/made flat by compression and extracted with an organic solvent using, in many cases, hexane of about 50 to 60° C. After that, the extract is filtered and the solvent is removed therefrom (by means of distillation in many cases) to give crude oil. The crude oil is further filtered or centrifuged to remove the insoluble fractions and water is added thereto to remove water-soluble substances (degumming) followed by carrying out the steps of deacidification, decolorization and deodorization to give a product. Usually, manufacture of lipids from a raw material containing high amount of lipids such as plant seeds is relatively easy (Non-Patent Document 1). On the other hand, in the case of animal raw materials, oil is usually separated out only by and heating of the raw material after washing. Therefore, the product is able to be more easily manufactured. In the case of fish for example, since a liquid part is spontaneously separated into an aqueous part and a crude oil when the raw material is boiled and squeezed, fish oil is able to be manufactured by purifying the oil (Non-Patent Document 2). However, the lipids prepared by the methods are mainly triglycerides.
It has been known that phospholipid, a class of the lipids, has health functions such as an improvement in fatty liver caused by choline deficiency, a reduction in LDL (bad cholesterol) in blood and an increase in HDL (good cholesterol) in blood. Additionally, improvement in neuropathy caused by hypertension and acetylcholine deficiency, promotion of absorption of oil-soluble vitamins, etc. is expected. Phospholipid has been mostly separated and purified from soybean seed or egg yolk. In the case from soybean seed, phospholipid is contained in a fraction which is removed as an insoluble matter during a degumming step in the manufacturing steps of soybean oil and it is decolorized and dried to obtain a phospholipid product called lecithin (Non-Patent Document 3). Soybean oil has a very big market and phospholipid as a by-product thereof is also produced in large quantities. In the case of egg yolk, since about one-third of its weight is lipids and about one-third of lipids are phospholipids, extraction and purification of phospholipid are relatively easy. However, in view of efficient extraction of phospholipid and maintenance of its stability, it is necessary that water is previously removed from the raw material egg yolk. Dried egg yolk is manufactured by inputting of heat cost and phospholipid is extracted therefrom.
With regard to other raw materials containing a relatively high amount of phospholipid, marine products such as marine fish egg and krill have been known. However, when those raw materials are compared with egg yolk, content of phospholipid in the raw material is low and other impurities such as organic acids are abundant. Therefore, purification is not easy. With regard to preparation of lipids from marine products, there have been various proposals up to now such as a method where the material is previously dried and then lipids are extracted therefrom, e.g., a raw material marine product is dried so as to make the water content 10% or less by weight and then lipids are extracted therefrom (Patent Document 1), a method where a raw material is dried by means of freeze-drying and then lipids are extracted therefrom (Patent Document 2), a method where an organic solvent is used, e.g., lipids are extracted from the raw material fish/shellfish with a mixed solution of acetone and water, (Patent Document 3) and a method where, in extraction of lipids from the starting krill, acetone is used as the first stage (Patent Document 4). However, any of those methods has problems such as difficulty in terms of cost, complicatedness in the steps and limitation in the use due to legal restrictions.
Highly unsaturated fatty acids have been known to have a preventing/improving activities for lifestyle-related diseases (such as arteriosclerosis, hyperlipemia and dementia) and an immunosuppressive activity (such as reduction in allergy and atopy). Furthermore, EPA is expected for its effect of prevention of circulatory diseases such as reduction in neutral fats and suppression of platelet aggregation while DHA is expected for its effect of growth and maintenance of function of nerve tissues and improvement in eyesight (Non-Patent Document 4). Lipids from marine products are promising not only as a supplying source for phospholipid but also as a supplying source for highly unsaturated fatty acids such as EPA and DHA. However, as mentioned above, marine products contain many impurities such as organic acids including amino acids and fatty acids. Therefore, extraction and purification of lipids therefrom have not been easy.
A method in which proteins are obtained by coagulating protein components in krill, which is a type of crustacean, by heating has been known in the past as a method for extracting proteins from krill (patent document 5). However, a coagulated product known as “Okean” obtained by this method was used in order to obtain proteins (non-patent document 5).
In addition, after the present application was filed, a method for obtaining a coagulated product by heating krill to 60 to 70° C. by means of hot water and then reheating the thus obtained aqueous solution of the supernatant liquid to 90° C. or higher was disclosed within the priority claim period (patent document 6).
Patent Document 1: JP08-325,192 A
Patent Document 2: JP2,909,508 B
Patent Document 3: JP2004-26767 A
Patent Document 4: WO 00/23546
Patent Document 5: SU227041
Patent Document 6: WO 09/027692
Non-Patent Document 1: “Saishin Shokuhin Kakou Kozo-Shokuyo Yushi to Sono Kako” edited by Tetsujiro Ohara, published by Kenpakusha, 1981, pages 49 to 74
Non-Patent Document 2: “Gyoyu to Maiwasi” edited by Hichiro Matsushita, published by Koseisha Koseikaku, 1991, pages 21 to 28
Non-Patent Document 3: “Bailey's Industrial Oil and Fat Products” edited by Y. H. Hui, published by John Wiley & Sons, 1996, Fifth Edition, Volume 1, page 336
Non-Patent Document 4: A. Clarke, Journal of Experimental Marine Biology and Ecology, 1980, Volume 43, No. 3, pages 221 to 236
Non-Patent Document 5: Vopr Pitan, No. 1, page 70 to 73, 1977